An aqueous dye ink composition comprising black dye has been mainly used as a recording liquid for a writing implement and an ink jet printer. Recently, notice has been made on an aqueous pigment ink composition in which a pigment such as a carbon black is included to get the light resistance and water resistance of the resulting recorded traces.
Carbon black is commercially available as a coloring agent, however, it generally shows insufficient affinity and dispersion stability to aqueous medium. Therefore, the carbon black is necessary to be dispersed and stabilized in an aqueous medium by using a dispersing machine, in the presence of what is called a pigment dispersing agent, such as an aqueous acrylic resin, a stylene-(meth)acrylic acid resin, and a stylene-maleic acid resin, or amine.
For example, in Japanese Patent Kokai Publication No. 6074/1989 and 31881/1989, there is described an aqueous pigment ink composition comprising pigment dispersing agents (an anionic surface active agent and a polymeric dispersant) and carbon black having a pH value of not more than 5. Further, in Japanese Patent Kokai Publication No. 210373/1991, there is described an ink composition for ink jet recording, comprising a water-soluble resin and acidic carbon black.
In general, for stable discharging of droplets from the minute point of an ink jet recording head, and for smooth writing with the fine pen tip of an aqueous ball-point pen, it is required to prevent setting of the ink at the orifice of the ink jet recording head or at the tip of the ball-point pen.
However, when commercially available carbon black is used, the resulting pigment ink composition have to comprise a pigment dispersing agent. A resin which constitutes the pigment dispersing agent easily adheres to an orifice and the like, and it hardly dissolves again, therefore, orifice-clog or discharge-failure of droplets often occurs.
Further, an aqueous pigment ink composition containing a pigment dispersing agent is viscous, and it makes resistance throughout the route leading to a nozzle tip, when high-speed printing, or continuous discharging for a long period of time is conducted. Therefore, the discharging becomes unstable, and smooth recording becomes difficult. Further, in the conventional aqueous pigment ink composition, the pigment concentration can not be increased sufficiently, because discharging stability have to be maintained, and consequently, density of the resulting recorded traces becomes insufficient by comparison with an aqueous dye ink composition.
In order to solve the above-described problems, the inventors provide an aqueous pigment ink composition comprising no or a reduced amount of pigment dispersing agent. In the aqueous pigment ink composition described herein, the water dispersibility of carbon black used as a coloring agent is remarkably improved, and a resinous component such as a pigment dispersing agent does not have to be included. Therefore, this aqueous pigment ink composition does not cause clogging in a nozzle when it is used for ink jet recording, enables smooth writing with a fine pen point, and provides sufficient density of recorded traces.